A Model for High-Temperature Pitting Corrosion in Nickel-Based Alloys Involving Internal Precipitation of Carbides, Oxid
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THE term high-temperature pitting corrosion is commonly used to describe local metal degradation that occurs due to the intrusion of reactive species from a hot surrounding medium following the breakdown of the protective surface oxide layer. Metal dusting is, thus, a specific form of this type of corrosion, which occurs during exposure to gas atmospheres containing H2, CO, CO2, H2O, and hydrocarbons within the temperature range 400 C to 800 C.[1,2] Iron-, cobalt-, and nickelbased alloys are prone to metal dusting and, usually, the J.Z. ALBERTSEN, formerly with the Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), is with StatoilHydro ASA, N-7005 Trondheim, Norway. Contact e-mail: [email protected] Ø. GRONG, Professor, is with the Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway. J.C. WALMSLEY, Senior Scientist, is with SINTEF Materials and Chemistry, N-7465 Trondheim, Norway. R.H. MATHIESEN, Professor, is with the Department of Physics, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway. W. VAN BEEK, Researcher, formerly with the Swiss-Norwegian Beamlines Grenoble, ESRF, F-38043 Grenoble Cedex, France, is also with the Dipartamento di Scienze e Tecnologie Avanzate and NanoSistemi IC, Universita’, del Piemonte Orientale Allessandria, I-15100 Alessandria, Italy. Manuscript submitted December 14, 2006. Article published online April 16, 2008 1258—VOLUME 39A, JUNE 2008
corrosion attack results in pitting and general metal loss in which the individual pits often merge to cause uniform metal wastage.[3] The early stages of attack are characterized by the formation of hemispherical pits containing a powdery mixture of carbides, oxides, carbon soot, and metal particles. At the same time, heavily carburized metal is observed beneath the pitted areas.[4] The phenomenon of metal dusting was first documented by Pattinson[5] in 1876, and similar exploratory reports were later published in 1945,[6] 1947,[7] and 1959.[8–10] In 1966, Hochman and Burson[11] studied the corrosion mechanism in more detail and laid the foundation for the current understanding of the phenomenon. Over the years, several researchers have contributed significantly to the development of the field,[12–36] but metal dusting corrosion is still a controversial research topic in the sense that the degradation mechanisms involved are not yet fully understood. Therefore, the subject is keenly debated in the scientific literature. In a real plant situation in which metal dusting occurs, a number of variables come into play that cannot readily be accounted for in a mathematical simulation of the corrosion process, e.g., alloy and gas composition, temperature, ambient pressure, exposure time, gas flow rate, and possible cyclic temperature variations.[33] In the past, the gas phase has mainly been characterized by its carbon activity, aC,[2,13,36–39] which METALLURGICAL AND MATERIALS TRANSACTIONS A
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